The usual jack-up offshore drilling rig includes a barge hull and supporting legs which are capable of being operated to jack up the hull above the surface of the water. One type of rig unit, generally known as the slot-type, provides a slot in the aft end of the hull and located inside of the peripheral boundary of said hull. The derrick and drilling equipment are permanently mounted above the slot so that the drill pipe which is suspended from the derrick in the usual manner extends through such slot. The drilling equipment which includes the rotary table and other necessary machinery is not capable of being skidded or moved outboard of the periphery of the drilling barge and therefore can never be employed to be extended outwardly to a position which is outboard of the vessel. For this reason, the slot-type rig cannot be employed for the purpose of either drilling a production well or for working over an existing well when said wells are located outside the peripheral boundary of the barge hull. One example of an offshore slot-type offshore drilling rig is illustrated in the patent to Samuelson Pat. No. 2,589,146.
A second type of jack-up offshore drilling rig is known as the cantilever type rig and includes a structure which can be skidded or moved outwardly beyond the peripheral boundary of the barge hull. By reason of being capable of such movement, the cantilever-type rig may be employed to drill exploratory wells or to be skidded out over an existing platform which is located beyond the hull periphery. The primary disadvantage of a cantilever-type rig is that in a storm or very severe weather conditions the extended cantilever structure must be retracted inwardly onto the upper surface of the barge. An example of prior art cantilever-type rigs are shown in the prior patent to Suderow Pat. No. 3,001,594 and in the photograph of a rig owned by the Crestwave Offshore Services, Inc. which appears on page 3930 of the Composite Catalog of Oilfield Equipment and Services, 1968-1969 edition of Volume 3.